SXAPS

AEAPS, SXAPS Auger electron APS, Soft x-ray APS Same as APS Same as APS... 

In SXAPS the X-ray photons emitted by the sample are detected, normally by letting them strike a photosensitive surface from which photoelectrons are collected, but also - with the advent of X-ray detectors of increased sensitivity - by direct detection. Above the X-ray emission threshold from a particular core level the excitation probability is a function of the densities of unoccupied electronic states. Because two electrons are involved, incident and the excited, the shape of the spectral structure is proportional to the self convolution of the unoccupied state densities. 

The principal advantages of AEAPS and DAPS over SXAPS is that they can be operated at much lower primary electron currents, thus causing less disturbance to any adsorbed species. 

Because IPES maps the densities of unoccupied states, it is related to other techniques that do the same (e.g. STS and SXAPS). When used in conjunction with a technique that maps the densities of occupied surface states, e.g. UPS or ELS, a continuous spectrum of state density from occupied to unoccupied can be obtained. Just as in UPS, in which angular resolution enables elucidation of the three-dimensional occupied band structure, so in IPES angular resolution enables mapping of the three-dimensional unoccupied band structure. This version is called KRIPES (i. e. K-re-solved IPES). 

The EAPFS excitation cross section is monitored by fluorescence from core - hole decay (also known as SXAPS). 

SXAPS Soft X-ray Appearance Potential Spectroscopy Another name for APXPS. 

There are three main detection modes for EAPFS within the appearance potential spectroscopy (APS) technique./31/ First, one may monitor soft-x-ray emission due to the decay of the core hole left by the primary process. This is called SXAPS-EAPFS (Figure le). Second, it is also possible to monitor Auger electrons due to the same core-hole decay, as in AEAPS-EAPFS and AMEFS-EAPFS, cf. Figure If. Third, one may measure the remaining total intensity of... 

Emission of photons Soft X-ray appearance spectroscopy SXAPS... 

Appearance potential spectroscopy involves detection of electronic transitions not of the backscattered electrons as in ELS, but of secondary processes. The latter include increase in soft X-ray (SXAPS) or Auger electron (AEAPS) emission or decrease in elastically scattered primary electrons (DAPS) (382). SXAPS is not as sensitive as AES for surface chemical analysis. However, SXAPS and IS spectra are easier to analyze than AES, since only one core transition is involved. This makes SXAPS and IS quite convenient for detecting heavy elements on catalyst surfaces. 

Soft X-ray appearance potential spectroscopy (SXAPS) studies of radiant coil alloys were carried out to determine surface compositions resulting from exposure to a range of environments. 

Coke deposits from a commercial ESC plant were also examined. Bulk metals concentrations were determined by ashing at 900°C prior to emission spectroscopy analysis. High levels of metals were found at the radiant coil-coke interface due, presumably, to oxide spallation, but concentrations of metals (primarily chromium, iron and titanium,totalling ca200 ppm) were also found into the bulk of the coke and at the coke-process stream interface (Table I). SXAPS examination of coke heated under vacuum for several hours at 900°C also revealed the appearance of up to 1% concentration of metals at the surface of some coke samples, and significantly chromium, iron and titanium were again the metals observed. 

Figure L SXAPS of Incoloy 800 alloy aged in a steam cracker.

Appearance potential spectroscopies (SXAPS, AEAPS, DAPS, TCS, XEAPS, EAPFS, lETS)... 

For SXAPS, the instrumentation is very simple. A filament is used as a source of electrons and is mounted close to the sample. The electrons from the filament are accelerated towards the sample. A screening mesh in front of the detector allows X-rays to pass into the detector, but not electrons. The X-rays cause photoelectrons to be emitted from the wall of the detector and these photoelectrons are collected on a thin wire. The detector has a superficial similarity to a Geiger-MuUer tube. UHV is not needed for the instrumentation (this is especially true for SXAPS), but a clean surface is necessary for reliable data. 

The change in the elastically scattered yield when the primary beam energy passes through an ionisation threshold is typically 0.1%. Hence, electronic differentiation is used to enhance the weak signal. AEAPS is approximately 10 times more sensitive than DAPS. SXAPS is approximately 10,000 times less sensitive than DAPS and hence is the least used despite its simple instrumentation. 

Other types of APS are x-ray photoelectron APS (XPAPS) and resonant photoelectron APS (RPAPS). In XPAPS, the sample is bombarded by photons [17,18]. The spectra obtained with this technique are much weaker than those of SXAPS and depend upon the material used for the anode of the x-ray tube. RPAPS, developed by Hua et al. [19], is a combination of SXAPS and XPAPS. In it the photocathode is made of the same material as the target. Using this, they have foimd RPAPS to be more sensitive to the sub-peaks for principal elements and peaks for the impurities present. However, these spectroscopies are not commonly used for surface analysis. 

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